Aircraft: MEDIUM TRANSPORT; HIGH WING; 2 TURBOPROP ENG
Phase of flight: Landing
Anomaly: Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Runway; Ground Incursion Taxiway

On arrival to BOI; I the flying pilot briefed the approach. The current ATIS was then cross referenced with NOTAMS on the release and the information stated that runway 10L had a Field Condition (FICON) score of 3/3/3 100 PCT 1/4IN WET SN. The Captain suggested that AUTO UPDATES be turned on due the fact that we were sending for ATIS updates since reaching cruise and it was observed that the conditions were changing frequently; but were improving. We ran landing numbers for 5/5/5 and 3/3/3 with Stall Protection Stall Speeds and had numbers that indicated we had sufficient runway to land safely with Autobrakes low and Thrust Reverse credit. The speeds were 136; 136; 151; 182 and 7;256FT FACTORED and 6;309 UNFACTORED (3/3/3); 136;136;151;182 6096FT FACTORED; 5301FT UNFACTORED (5/5/5); so Autobrakes Low were decided which indicated 9038FT (3/3/3); and 9269FT (5/5/5). Autobrakes LOW was also selected to prevent any skidding or loss of control the aircraft and it was decided to mitigate the runway conditions to not disengage the autobrakes until a low and safe speed was obtained. The Captain also pointed out that the end of runways are often more slippery and that braking should be accomplished to slow the aircraft to a safe taxi speed prior to last quarter of the runway. The KYAAN 2 Arrival was briefed and runway 28R was also briefed for an ILS Landing due to current conditions. Threats were identified to be the visibility; we mitigated by deciding to keep autopilot on till the runway environment was in site Once on the arrival a flight was in front of us had landed on runway 28R approximately 5 minutes before us and reported the runway conditions to be medium. Were in IMC conditions intercepting the localizer and was VMC at ZIXIP; which was at 6000 feet; 5 miles out and the runway was visible. On short final; the tower gave wind speeds to be 120 at 4 knots. ATIS information was also updated; but due to the phase of flight; positive visual of the runway the ATIS updates were not checked. Autopilot was disengaged 200 feet above minimums; which is approximately 500 feet; 3300 feet indicated. The runway environment was clearly visible and the touchdown area was visible and power was removed at 30 feet and flare initiated. Touchdown occurred within the touchdown zone and the thrust reversers were applied to max. The thrust reversers were continued to be used since they were not causing white out conditions. Due to the number of taxiway closures; the only two options to exit the runway were taxiway D and taxiway W. Near taxiway Gulf the centerline was very difficult to see and the captain took over the controls because the aircraft was at 80 knots. At taxiway Gulf there was approximately 2;000 left of usable runway. The Captain stowed the thrust reverses at 60 knots just past taxiway Gulf due to good braking and the aircraft was slowing safety and the aircraft was going to stop far short of taxiway whiskey so the autobrakes were overridden and thrust reversers stowed. As we approached taxiway W; the aircraft was slowed to a slow taxi speed. No speed was not noted on the tape. The captain applied brakes to slow even more; which was approximately 200 feet before taxiway Whiskey. Anti-skid braking was engaged and working; but did not slow the aircraft due to conditions on the runway near taxiway Whiskey; which was not 5;5;5 as reported. Runway markings were not visual. The aircraft did not slow due to brake applications and the speed at this time was no more than 10 knots. Once again; no visual runway markings were noted past taxi way Gulf. The conditions made stopping and turning difficult at any speed. With brakes still being applied; the aircraft continued into the closed taxiway Zulu; which was not plowed and had approximately 6 to 8 inches of snow on it. The aircraft gently went in to the unplowed snow and stopped softly and gently. There was no jerking or sudden stopping. The aft flight attendant noted that she was hardly aware the aircraft stopped. After the aircraft came to a stop; we noted that the thrust reversers deployed caution messages were deployed and that the safest action was to request a tow from the taxiway. The Captain did redeploy the thrust reversers prior to enter the closed taxiway while still on the runway at the last minute; but restowed them when it was clear it was not going to help. There was a cargo plane behind us and I think that caused the Captain to rush to get off the runway. After the aircraft arrived at the gate I performed a very slow and careful walk around to assure the aircraft was not damaged. I did not notice any damage and was sure to check under the engine cowlings. The main contributing factor to this event was poor weather and record snow fall. The weather made the end of runway 28R; approximately last 1;000 feet; contaminated with poor braking conditions. The last 500 feet of the runway had braking conditions that were NILL. I also feel that Big Sky Arrival put the plane too close behind us in sequence and did not consider runway conditions and the extra time it takes for planes to exit a runway with only two taxi ways cleared for exit. The fact the airport was experiencing record snow fall the night of the incident and ground operations were working as fast and as hard as they could; the only way to avoid this is to keep the runway free of contaminates allowing for better braking. Due to the fact snow is falling faster than the ground equipment could keep up; maybe operating one runway in poor conditions that would have an ILS from either direction would allow for equipment and ground operation crews to keep up with the weather and avoid having runways going such long period of times between cleaning. Maybe the purchase of more snow removal equipment; but due to the fact this is a record snow fall year; and is this not a common occurrence; this would result in a costly expenditure that may not be feasible for the City of Boise. [Report narrative contained no additional information.] Air Carrier flight crew reported being unable to stop before the end of Runway 28R at BOI due to nil braking action in the last 200 feet of the runway.